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In this paper we present the results of experimental investigation of wave-front distortion by amsotropic Te02 Bragg cellwith off-axial geometry. The measurements have been carried out using Shack-Hartmann wave-front sensor at visiblewavelength (O.63mkm) over the frequency range from 85 to 1 15MHz. We have not noticed the significant dependence ofphase distortion on frequency, nevertheless, they are in significant excess of measurement errors.Keywords: wave-front, aberrations, Shack Hartmann sensor, Bragg cell. 1. INTRODUCTION Uniform laser beam irradiation is needed in a wide range of applications. On the other hand, the diffraction oflight wave on the inhomogeneities of propagation channel may introduce a rapidly varying distortion in phase distributionof light beam, which affects the formerly plane wave-front during propagation. Irregular wave-front perturbations in thephotodetector plane of a coherent processing system, if exist, can result in considerable degradation of an output signal.Visualization of a phase structure of sound field in a Bragg cell made earlier using differential-phase technique,1 showedunexpectedly great phase perturbations of diffracted beam even at the only frequency, the fact having no explanation. Forthis reason it is of great interest to investigate its uniformity over all frequency range of a Bragg cell by another technique.All approaches to the measuring and real-time correction of those distortions require a direct knowledge of the form of thewave-front, and postprocessing algorithms to utilize received data. In this paper we present an analysis of wave-frontsensing, having a common use in adaptive optics, as applied to phase distortions of acousto-optic devices (AOD).It is well known that the resolution of AOD is really restricted not only by the diffraction limit of its aperture, butalso by the distortions of light wave in AOD. These distortions have their origin in defects of acousto-optical crystal,acoustic field inhomogeneity, heat effects, etc. To increase the resolution up to diffraction limit we can eliminate theirregularities of the wave-front of diffracted wave by adaptive optics or holographic methods. For this reason the problemof AOD wave-front measurement and correction is actual for optical data storage and read-out.A first technique could be to retrieve the phase p from the intensity distribution in the focal plane. This distributionfor a point source and a monochromatic beam is the point spread function which is the Fourier transform of the opticaltransfer function. This inverse problem is so-called phase problem in optics.2 There is no unique solution in the generalcase, and measurements and/or a priori constrains must be used. The Gershbery-Saxton iterative algorithm is the basicprinciple of the technique.3 The main drawbacks of these methods are the requirement of monochromatic point sources,and the computing time.The most popular optical techniques to sense the wave-front is based on the methods used in interferornetric opticaltesting. There is a lot of methods based on interferometry. The principle is to form a fringe pattern between the beamcoming from the test object and the beam coming from the reference mirror. The main disadvantage of the device in aBragg cell testing is the requirement for the identity of the frequencies of reference and tested beams. Because of frequencyshifting of diffracted wave, observed fringe pattern is unstable, and its registration is made difficult. In addition, therestoration of phase distribution is complicated by speckle in formed interferograrn. |
